An apparatus for detecting tracking and focusing errors in an optical recording system is described in Japanese laid open No. 38742/85 which was laid open on Feb. 28, 1985. That apparatus for detecting tracking and focusing errors in an optical recording system comprises a prism in which an upper half portion of light beam reflected from an optical disk is received and from which two-divided light beams radiate with predetermined symmetrical angles in regard to the axis of the incident light beam, two-separated photodiodes which receive the two-divided light beams respectively, and two-divided photodiodes in which a lower half portion of the light beam reflected from the optical disk and not passed through the prism is received.
In operation, the tracking error is detected in accordance with the comparison in level between the two outputs of the two-separated photodiodes while the focusing error is detected in accordance with the two outputs of the two-divided photodiodes.
In the apparatus for detecting tracking and focusing errors in an optical recording system as mentioned above, the construction thereof becomes smaller and simpler for the reason that the separated and divided photodiodes are positioned on a plane of a single photodetector.
According to that apparatus of detecting tracking and focusing errors in an optical recording system, however, the sensitivity of the detection thereof is not as high as expected for the reason that the diffraction light beams passing through the prism for detecting the tracking error therein are only half the light beam reflected from the optical disk.
On the other hand, there has been practically used an apparatus for detecting tracking and focusing errors only, that apparatus being adapted for use in an optical reproducing system which comprises a lens for focusing a light beam reflected from three spots on an optical disk, a cylindrical lens in which the light beam is received and from which three-divided light beams radiate with three different angles, and a photodetector including four-divided photodiodes in contact with each other and two-separated photodiodes positioned at outer sides thereof.
In operation, there are formed three spots on the optical disk, for instance, by diffraction gratings wherein one of the spots is positioned on a track on the optical disk while the two remaining spots are positioned at the right and left edges of the track respectively. The light beam reflected from the three spots on the optical disk is introduced through the focusing lens to the cylindrical lens to be divided into three light beams. The central light beam of the three-divided light beams is received in the four-divided photodiodes of the photodetector whereby the focusing error is detected in accordance with the astigmatism method to be described later in more detail, while the two remaining light beams at the outer sides of the central light beam are received in the two-separated photodiodes of the photodetector whereby the tracking error is detected in accordance with the comparison in level of the two outputs therefrom.
Even in such apparatus for detecting tracking and focusing errors only in an optical reproducing system, the construction thereof becomes smaller for the reason that the tracking and focusing errors are detected by a single photodetector having the separated and divided photodiodes.
However, the stability of detecting the tracking error is decreased in the apparatus mentioned above for the reason that the two spots are formed at the front and rear positions on the optical disk in regard to the central spot, thereby detecting the tracking error so that the reflection factor is changed at the front and rear positions on the optical disk when the recording operation is performed thereon by the central spot. A further disadvantage in the apparatus mentioned above is that the focusing error signal is affected by the tracking error signal for the reason that the detection of the focusing error is performed in accordance with the far-field in the astigmatism method as compared to the knife-edge method wherein the error is detected in accordance with a spot to be focused.